Expansion

Most substances expand when they are heated and contract when they are cooled. Heat causes expansion because it increases the vibrations of the atoms or molecules of a substance. The increased vibration forces the atoms or molecules further apart and the substance becomes larger, i.e. expands. When the substance cools down, it becomes smaller, i.e. contracts.

Expansion and Contraction of Solids

Solids expand in all directions when heated but the amount of expansion is usually very small.
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The above experiments show that: after heating, the metal expands and will not fit into the gauge in either length or diameter; after heating, the ring expands an will let the ball pass through it; when the metal rod is heated, the pointer is pushed to the right indicating expansion.

Force of Expansion and Contraction
If there is an obstruction to the free movement of an expanding or contracting body, very large forces may be set up. These forces can be useful but they can be destructive as well.

The steel bar in Figure 2 is held by a cast iron pin at one end. When the bar is heated, it expands and the nut is tightened to ensure the rod is firmly held. The rod is then allowed to cool snapping the pin as it contracts.

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Expansion of Various Solids
When rods of the same length but different substances are heated through the same increase in temperature, they expand by different amounts.

Table 1 shows the expansion of metre –long rods of different substances which are heated form 0oC to 30oC.
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Applications of the Expansion of Solids
Rivets
Rivets are steel bolts used to fasten steel plates firmly together. The rivet is heated to a very high temperature before being fitted through holes in the steel plates. The thin end of the rivet is thickened by hammering. ON cooling the rivet contracts and pulls the plates